The use of renewable energy sources is becoming increasingly necessary to reduce emissions of carbon based fuels and provide alternatives to petroleum based energy and feedstocks. One of the alternatives being explored is the use of biomass. Biomass is any carbon containing material derived from living (or formerly living) organisms, such as wood, wood waste, crops, crop waste, waste, and animal waste.
Pyrolysis, which is the thermal decomposition of a substance into its elemental components and/or smaller molecules, is used in various methods developed for producing hydrocarbons, including but not limited to hydrocarbon fuels, from biomass. Pyrolysis requires moderate temperatures, generally greater than about 325° C., such that the feed material is sufficiently decomposed to produce products which may be used as hydrocarbon building blocks.
The pyrolysis of biomass generally produces four primary products, namely water, “bio-oil,” also known as “pyrolysis oil,” char, and various gases (H2, CO, CO2, CH4, and other light hydrocarbons) that do not condense, except under extreme conditions. For exemplary purposes, the pyrolysis decomposition products of wood from white spruce and poplar trees are shown in Table 1.
TABLE 1Source: Piskorz, J., et al. In Pyrolysis Oilsfrom Biomass, Soltes, E. J., Milne, T. A., WhiteEds., ACS Symposium Series 376, 1988.SprucePoplarMoisture content, wt %7.03.3 Particle size, μm (max)1000590Temperature500497Apparent residence time0.650.48Product Yields, wt %, m.f.Water11.612.2Gas7.810.8Bio-char12.27.7Bio-oil66.565.7Bio-oil composition, wt %, m.f.Saccharides3.32.4Anhydrosugars6.56.8Aldehydes10.114.0Furans0.35—Ketones1.241.4Alcohols2.01.2Carboxylic acids11.08.5Water-Soluble-Total Above34.534.3Pyrolytic Lignin20.616.2Unaccounted fraction11.415.2
Fast pyrolysis is one method for the conversion of biomass to bio-oil. Fast pyrolysis is the rapid thermal decomposition of organic compounds in the absence of atmospheric or added oxygen to produce liquids, char, and gas. Generally, fast pyrolysis uses <10% dry feedstock of biomass comminuted into small particles (< about 3 mm), moderate temperatures (325-750° C.), and short residence times (0.5-2 seconds). This pyrolysis reaction may be followed by rapid quenching to avoid further decomposition of the pyrolysis products and secondary reactions amongst the pyrolysis products.
Fast pyrolysis affords operation at atmospheric pressure, moderate temperatures, and with low or no water usage. Bio-oil yields typically range from 50-75% mass of input biomass and are heavily feedstock dependent. Generally, known methods of bio-oil production result in bio-oil with high oxygen and water content, and the high oxygen and water content may result in storage instability and phase-separation issues.
For example, the pyrolysis of a wood-based biomass will produce a mixture of organic compounds such as lignin fragments, aldehydes, carboxylic acids, phenols, furfurals, alcohols, and ketones, as well as water. Unfortunately, compounds such as the aldehydes and acids may react with other components of the bio-oil, creating instability, corrosiveness, and poor combustion characteristics.
Other compounds in the bio-oil, such as alcohols, carbonyls, carbohydrates, and phenols have potential in the industry as commodity chemicals, but only if they can be purified in a cost effective manner. Therefore, the ability to selectively remove compounds from bio-oil into defined fractions is important to fully develop biomass as a renewable resource.
Many known biomass fast pyrolysis processes use liquid coolants injected into spray scrubbers to rapidly cool bio-oil vapors into a liquid form, but without attempting to recover individual chemical components. The result is a complex mixture that is acidic and unstable, requiring costly and/or complicated methods to stabilize the bio-oil for storage and subsequent use.
Other known processes use liquid cooled indirect contact heat exchangers to recover bio-oil. One process, disclosed in U.S. Patent Application Ser. No. 61/093,045, uses a combination of condensers with finely controlled wall temperatures and electrostatic precipitators to selectively condense and collect pyrolysis product vapors based upon partial pressures and saturation temperatures. While this method selectively condenses and collects bio-oil vapors into unique fractions, the approach requires a unique condenser for each fraction collected and is thus expensive to implement. In addition, this method is subject to charring and tarring that leads to system clogging and secondary reactions.
Therefore, it would be desirable to have a method of separating the various hot pyrolysis products into useful fractions in a more cost effective manner.